A known conventional process for preparing aromatic carbonate esters carries out a transesterification between an aliphatic carbonate ester and an aromatic hydroxy compound or aromatic carboxylate wherein an alkoxyl group is an aromatic group. In particular, with respect to a process for preparing diphenyl carbonate by reacting a dimethyl carbonate as one type of aliphatic carbonate esters with phenol as one type of aromatic hydroxy compounds by way of methylphenyl carbonate, various proposals have been made. For example, the following four publications disclose known processes for preparing diphenyl carbonate. U.S. Pat. No. 4,045,464 uses Lewis acid or a compound producing Lewis acid as a catalyst. U.S. Pat. No. 4,182,726 uses a titanium compound or an aluminum compound as a catalyst. Japanese Publication for Examined Patent Application No. 3181/1989 (Tokukosho 64-3181) uses a lead compound as a catalyst. Japanese Publication for Unexamined Patent Application No. 48733/1979 (Tokukaisho 54-48733) uses an organotin compound as a catalyst.
All of the above-mentioned processes are carried out using a batch system, and methanol as a by-product is distilled off while supplying dimethyl carbonate to a reaction system. Thus, this process suffers from drawbacks including a long reaction time and low productivity. Moreover, in order to efficiently distill off methanol, a known process (Japanese Publication for Examined Patent Application No. 8091/1987 (Tokukosho 62-8091)) adds benzene to the reaction system. However, this process requires a complicated collection operation, for example, extracting methanol from a mixed solution of benzene and the methanol using water, when collecting the methanol. Thus, this process also failed to achieve sufficiently high productivity.
U.S. Pat. No. 5,210,268 discloses a process for preparing methylphenyl carbonate by reacting dimethyl carbonate and phenol in a continuous multistage distillation column. In this process, however, the conversion of dimethyl carbonate is about 1.6 mole percent to 24 mole percent, resulting in low productivity. Additionally, in this process, an object, diphenyl carbonate, is obtained by a disproportionation of methylphenyl carbonate, and therefore an increased number of processes is required to obtain the diphenyl carbonate compared with the above-mentioned processes.
The reason why the conversion of dimethyl carbonate is low is as follows. A transesterification reaction for producing methylphenyl carbonate is an equilibrium reaction (equilibrium constant K=10.sup.-3 to 10.sup.-4) in which the equilibrium is extremely biased toward the original system and the reaction substantially does not progress.
U.S. Pat. No. 4,533,504 discloses a process for preparing diphenyl carbonate by reacting dimethyl carbonate and phenyl acetate. This process achieves high conversion of dimethyl carbonate, not lower than 70 mole percent. However, in this process, a batch system is used, and it is necessary to convert methyl acetate to diketene at very high temperatures and react the diketene with phenol when reproducing phenyl acetate from the by-produced methyl acetate. Consequently, the step of reproducing phenyl acetate gives a low yield, and the cost of service needs to be covered.
As described above, the conventional processes suffer from drawbacks including a low conversion and inefficient syntheses of raw materials. In the conventional process for preparing diphenyl carbonate by reacting dimethyl carbonate with phenol, since the dimethyl carbonate and the by-produced methanol form an azeotrope, it is difficult to separate them. In short, with this conventional process, it is impossible to efficiently prepare carbonate esters.
Then, there is a demand for a process capable of industrially preparing carbonate esters in an efficient manner.
With respect to industrial processes for preparing aromatic carboxylates, for example, the following processes are known. A process for esterifying aliphatic carboxylates with aromatic hydroxy compounds (Japanese Publication for Unexamined Patent Application No. 36131/1973 (Tokukaisho 48-36131) and No. 54525/1976 (Tokukaisho 51-54525)). A process for reacting isopropenyl acetate and phenol (Tetrahedron Letters, Vol. 29, No. 36, p.4567-4568, 1988). A process of esterification with highly reactive raw materials such as diketene (U.S. Pat. No. 4,533,504).
However, with these conventional processes, aromatic carboxylates can not be efficiently prepared because the conversion is relatively low or the syntheses of raw materials is difficult. A process for industrially preparing aromatic carboxylates by a transesterification reaction between a suitable aliphatic carboxylate and aromatic hydroxy compound has not been known. The reason for this is that the transesterification reaction is an equilibrium reaction (equilibrium constant K=10.sup.-3 to 10.sup.-4) in which the equilibrium is extremely biased toward the original system, the equilibrium conversion is not higher than several mole percent, and the reaction substantially does not progress.